Faced with revolts throughout his reign, before he died he made his wife regent for his son, who became Ferdinand IV.

Sancho was recognised and supported by the majority of the nobility and the cities, but a sizable minority opposed him throughout his reign and worked for the heirs of Ferdinand de la Cerda.

One of the leaders of the opposition was his brother John of Castile, who united to his cause the lord of Biscay, Lope Díaz III de Haro.

According to the chroniclers, he cemented his hold on power by executing 4,000 other followers of Infante Alfonso, son of Ferdinand de la Cerda, in Badajoz.

Just before succumbing to a fatal illness (possibly tuberculosis)[2] he appointed his wife, María de Molina, to act as regent for his nine-year-old son, Ferdinand IV.